Can I delete invoices from all sales page especially unapplied status ones?

Thanks for taking the time to post here in the Community, @Flex1. 

I'll help ensure you're able to delete the unapplied invoices for March from the All Sales page.

If you want to delete multiple invoices, this is currently unavailable. However, you can delete them on an individual basis. Before performing the deletion steps below, I'd suggest considering the following reminders: 

• Deleting removes the transaction completely and can change amounts for prior periods.
• There isn't an option to restore deleted transactions. You'll need to use the Audit Log in case you'll need to record them again in the future. 
• It can cause accounting issues if the deleted transactions are linked to other transactions. 
• The next reconciliation beginning balance will change if you delete transactions that were previously reconciled. 

Then, let's proceed on how to delete them: 

1. Go to Sales from the left menu. 
2. Select the All Sales tab. 
3. Choose Unbilled, Unpaid and Open Invoices section. 
4. Go to the Action column on each unapplied invoice. 
5. Click Delete from the drop-down.

For more information, you can check out this helpful article: Delete An Invoice. To learn more about the VAT filing periods, here's a great article you can refer to: VAT Guide. 

The steps and resources above will help delete your unapplied invoices so you can start fresh from April 1st, @Flex1. 

I'm here anytime you need anything else. Have an awesome day.